The Washington area's plans to expand the Metro subway system became embroiled in a bitter dispute yesterday as Prince George's County officials threatened to go to court to block rail construction projects.

The threat, disclosed by county members of the Metro board, was triggered by Metro officials' moves to spend millions of dollars in federal funds to build subway stations in the District for the long-delayed Green Line. County officials charged that the actions could impede Metro work in Prince George's.

In addition, county officials threatened to prevent the transit authority from adopting a new short-term plan for expanding the rail system to 84.7 miles by 1992. It is currently 60.5 miles long. They denounced the plan as reflecting needless and risky concessions to financial prodding from the Reagan administration.

"It's a wimpy plan," said Robert B. Ostrom, a Prince George's representative on the Metro board. "I don't understand why this board has such a lack of resolve."

The dispute, which pitted Prince George's against District and Virginia officials, marred the subway system's 10th anniversary yesterday. A festive "birthday party" has been scheduled by the authority for 11 a.m. Saturday at the Pavilion at the Old Post Office, 1100 Pennsylvania Ave. NW.

Yesterday's conflict stemmed from recent moves by the Reagan administration to curtail federal spending for Metro. The administration has refused to release nearly $400 million in funds previously appropriated by Congress and has recommended halting appropriations for Metro after the current fiscal year.

The 84.7-mile plan, made public yesterday, was drawn up in an attempt to persuade the administration to release the funds. Under the proposal, a section of the Green Line between Anacostia and a U Street Station in Northwest Washington would open in 1990, along with a Red Line extension to Wheaton and a Yellow Line spur to Alexandria's West End.

The plan provides for extending the Green Line from Fort Totten in Northeast Washington to Prince George's Plaza in 1992. These sections would be financed by previous federal appropriations and local funds. Other parts of the planned 103-mile system would be completed only if new funds become available.

The plan was approved "in principle" yesterday by the Metro board over Prince George's County's objections but, officials said, it cannot be formally adopted without ratification by Prince George's and other local governments. Federal officials said they have not reviewed the proposal.

Overriding protests by Prince George's representatives, the board also voted to accept a $40.6 million federal grant for work on the Shaw station near Seventh and R streets NW. It awarded a $44.3 million contract for construction of the U Street station between 10th and 13th streets NW.

In December, the board had rejected a $42.7 million bid on the U Street project, contending that the firm failed to comply with rules governing participation by minority-owned businesses. The three-year project, which includes a 1,830-foot tunnel, is expected to result in traffic disruption near U Street and Vermont Avenue NW.